3 ms·

Compelling analysis. But if hail damage is the primary concern, tracking seems like an X/Y solution. What about eg anti hail netting that can be rolled out when

by nbadg 1y ago

Compelling analysis. But if hail damage is the primary concern, tracking seems like an X/Y solution. What about eg anti hail netting that can be rolled out when the weather forecast predicts hail? It seems to me like a middle ground might be automated deployment of hail protection, which by definition has lower aero loading than what amounts to a big semiconductor sail.

dylan604 1y ago
anti-hail netting? you must not be from somewhere that receives a lot of hail. hail punches through solid coverings at the right size. if it's not the right size, then it's not that damaging. netting? ha! thanks for the funny visual so early in the morning.
- jfengel 1y ago
  Sorry, I don't get the joke. Nets can be pretty strong. Is it because there's a lot of hail at sizes big enough to do damage but too small for a net to catch without deep shadows?
- quesera 1y ago
  I think a hail net would be feasible for some cases. Golf balls break solid coverings too (and windows, car roofs, etc), but we have nets for them. You want high tensile strength and some level of flexibility/elasticity to absorb the hailstone energy over a greater-than-zero distance. Or to shatter the hailstone well above the solar panel. Probably too light-blocking to leave up continuously, and maybe awkward and failure-prone to deploy automatically. So insurance is probably more cost-effective for most installations.
  - medoc 1y ago
    Anti-hail netting is definitely a thing (protecting cars or fruit trees), for the reasons you state. Even big hailstones are rather slower than fast golf balls.
    - dylan604 1y ago
      Hail damage at car dealerships in my area is so expensive that the dealers have added coverings for the lots (pretty sure highly encouraged by the insurance companies), and they are most definitely not nets. Nets would have been cheaper, so that should say something about their effectiveness (or not)
      - b112 1y ago
        Ascetically, nets get dirty and are hard to clean, have birds on them, things grow on them, and they look gaudy to some.
      - nbadg 1y ago
        Hail protection nets are a COTS product. Full stop. Properly-designed nets are exceptionally strong; that's why we make "bulletproof" vests out of them (that's all they are, just a fiber net with a very tight weave). Just because your local car dealership(s) didn't opt for hail protection nets isn't on its own evidence for or against their effectiveness. There are a great many factors that go into such a decision and, unless you were privy to the decisionmaking process itself, whether or not nets were considered -- much less their effectiveness -- is pure speculation on your part.
  - nbadg 1y ago
    I agree with your implicit point that the key question is probably cost effectiveness compared with insurance. However, in the context of the OP, insurance premiums in hail-prone regions was listed as a prime reason why single-axis-tracking panel installations are, in some regions, probably still a better bet at grid scale. The point I'm trying to make is just that there are a lot of off-the-shelf (or nearly so) hail protection strategies that seem like they might have better economics than single-axis-tracking installations, which might improve the cost effectiveness of a fixed panel installation to the point where, even in hail-prone regions, it might be doable. To be clear: anti-hail nets are already a COTS product, just not for this application (as far as I'm aware). Nets are just really, really good at absorbing kinetic energy. I mean, that's effectively what kevlar ("bulletproof") vests are made of -- netting with a very small cell size. But it seems to me like some kind of roll-out kevlar or dyneema netting could be really effective at protecting panels, though I think you'd need some kind of better strategy for safely dispersing the hailstones after they were caught, so you don't end up with a huge collection of hailstones on the net. I agree that there's probably too much light blocking to leave them up continuously, and I'd also be worried about UV damage. But remember that the comparison here is to single-axis-tracking installations; I think automatic deployment of anti-hail netting could easily be made at least as reliable as the tracking system, if not significantly more so. Maintenance and testing would also be very cheap in comparison, since you could do it at night (ie without affecting power production), and damage to the protection system could be repaired without taking panels offline. The more I think about it, the more I'd be interested in seeing it deployed.
- tzs 1y ago
  Hail punches through solid coverings because they are rigid. They nearly instantaneously stop the hail which means they absorb all of the kinetic energy of the hail very quickly. They don't have a good way to dissipate that energy as rapidly and it goes into breaking the structure of the covering. Nets are flexible. The energy of the collision goes into accelerating the net at the point of the collision. The net can quickly spread that energy into accelerating the surrounding areas of the net and so on. Unlike with a rigid cover this is not nearly instantaneous, so you don't have all the kinetic energy of the hail being poured into the net at once. That and the ability of the net to rapidly spread energy means that you don't get enough energy anywhere to break the net. It just stretches as it decelerates the hail. You can see how this works by watching a soccer game and observing how the nets at the goals stop balls. Hailstones that weigh more than a soccer ball are extremely rare, as are hailstones that are falling faster than many strikes and penalty kicks in a professional soccer game, and you don't see many soccer balls breaking through the net. People have calculated how fast a soccer ball would have to go to get through the net and found that it is over 220 mph. The largest recorded hailstone is estimated to have had a terminal velocity of 168 mph, based on size, mass, and atmospheric conditions. That hailstone had about 16% more kinetic energy than a 220 mph soccer ball and so might have broke the net, but soccer nets are by no means the strongest nets that can made. And remember that was the largest hailstone ever recorded--we are talking a once in decades event.
Calwestjobs 1y ago
i used standard chain link fence on top of my small greenhouse and it works even with such big "holes", no mechanism, just manual labor but needs good forecast which can be issue.
- nbadg 1y ago
  Chain link fence is a really clever solution!
  - Calwestjobs 1y ago
    well, makes goats not eat precious plants so if it can withstand them, then hail will be ez. XD (neighbors goats. i do not like animals, too much worry and work around diseases. there are tens, some years even hundred millions of chicken burned because of avian flu, pigs have african swine fewer,.... no thank you, corona cave bats of 2019, was enough for me...)
- kyboren 1y ago
  > it works even with such big "holes" I'm intrigued. Can you expand more on this? Why does it work? I guess because the hail is rarely coming straight-on at the holes and instead makes more glancing blows off the wire? In any case I think it's a perfect example of a good hack. Original out-of-the-box thinking producing an effective but unconventional solution. Chapeau bas!
  - Calwestjobs 1y ago
    i saw chicken wire used for something unknown to me on top of old 1800s greenhouses in France. it was not used for hail because it was resting directly on top of glass. i do not know what it was there for. but hail and flying debris i got past few years is so big and so frequent that im not sure chicken wire will do the job. a lot of gardeners use shading cloth for lowering sun light intensity. but that is not always enough to withstand hail. A lot of times there is multiple pieces of hail stuck in one hole. some just bounce off. i have PV on roof and not had issue with that. but i had few broken glass pieces of greenhouse glass. even tho it is on same property. it is glass-glass bifacial, i did not do it for back light but for longevity, i saw too many plastic backing tear off, or delaminated or how to call it. so i did not want that. and im not sure if that is also factor in not having broken PV. doing it on huge PV areas should atleast be by my feeling too much of hassle, investment. But some people mentioned in other comments small arrays of, if i remember correctly, 5kWp. so that is roughly area im doing it on. so in those small arrays it can be done manually. https://99percentinvisible.org/article/fruit-walls-before-greenhouses-walled-gardens-created-urban-micro-climates/ https://99percentinvisible.org/article/fruit-walls-before-gr... https://www.atlasobscura.com/adventures/trips/peru-machu-picchu https://www.atlasobscura.com/adventures/trips/peru-machu-pic...

by nbadg 1y ago

dylan604 1y ago

anti-hail netting? you must not be from somewhere that receives a lot of hail. hail punches through solid coverings at the right size. if it's not the right size, then it's not that damaging. netting? ha! thanks for the funny visual so early in the morning.

jfengel 1y ago
Sorry, I don't get the joke. Nets can be pretty strong. Is it because there's a lot of hail at sizes big enough to do damage but too small for a net to catch without deep shadows?
quesera 1y ago
I think a hail net would be feasible for some cases. Golf balls break solid coverings too (and windows, car roofs, etc), but we have nets for them. You want high tensile strength and some level of flexibility/elasticity to absorb the hailstone energy over a greater-than-zero distance. Or to shatter the hailstone well above the solar panel. Probably too light-blocking to leave up continuously, and maybe awkward and failure-prone to deploy automatically. So insurance is probably more cost-effective for most installations.
- medoc 1y ago
  Anti-hail netting is definitely a thing (protecting cars or fruit trees), for the reasons you state. Even big hailstones are rather slower than fast golf balls.
  - dylan604 1y ago
    Hail damage at car dealerships in my area is so expensive that the dealers have added coverings for the lots (pretty sure highly encouraged by the insurance companies), and they are most definitely not nets. Nets would have been cheaper, so that should say something about their effectiveness (or not)
    - b112 1y ago
      Ascetically, nets get dirty and are hard to clean, have birds on them, things grow on them, and they look gaudy to some.
    - nbadg 1y ago
      Hail protection nets are a COTS product. Full stop. Properly-designed nets are exceptionally strong; that's why we make "bulletproof" vests out of them (that's all they are, just a fiber net with a very tight weave). Just because your local car dealership(s) didn't opt for hail protection nets isn't on its own evidence for or against their effectiveness. There are a great many factors that go into such a decision and, unless you were privy to the decisionmaking process itself, whether or not nets were considered -- much less their effectiveness -- is pure speculation on your part.
- nbadg 1y ago
  I agree with your implicit point that the key question is probably cost effectiveness compared with insurance. However, in the context of the OP, insurance premiums in hail-prone regions was listed as a prime reason why single-axis-tracking panel installations are, in some regions, probably still a better bet at grid scale. The point I'm trying to make is just that there are a lot of off-the-shelf (or nearly so) hail protection strategies that seem like they might have better economics than single-axis-tracking installations, which might improve the cost effectiveness of a fixed panel installation to the point where, even in hail-prone regions, it might be doable. To be clear: anti-hail nets are already a COTS product, just not for this application (as far as I'm aware). Nets are just really, really good at absorbing kinetic energy. I mean, that's effectively what kevlar ("bulletproof") vests are made of -- netting with a very small cell size. But it seems to me like some kind of roll-out kevlar or dyneema netting could be really effective at protecting panels, though I think you'd need some kind of better strategy for safely dispersing the hailstones after they were caught, so you don't end up with a huge collection of hailstones on the net. I agree that there's probably too much light blocking to leave them up continuously, and I'd also be worried about UV damage. But remember that the comparison here is to single-axis-tracking installations; I think automatic deployment of anti-hail netting could easily be made at least as reliable as the tracking system, if not significantly more so. Maintenance and testing would also be very cheap in comparison, since you could do it at night (ie without affecting power production), and damage to the protection system could be repaired without taking panels offline. The more I think about it, the more I'd be interested in seeing it deployed.
tzs 1y ago
Hail punches through solid coverings because they are rigid. They nearly instantaneously stop the hail which means they absorb all of the kinetic energy of the hail very quickly. They don't have a good way to dissipate that energy as rapidly and it goes into breaking the structure of the covering. Nets are flexible. The energy of the collision goes into accelerating the net at the point of the collision. The net can quickly spread that energy into accelerating the surrounding areas of the net and so on. Unlike with a rigid cover this is not nearly instantaneous, so you don't have all the kinetic energy of the hail being poured into the net at once. That and the ability of the net to rapidly spread energy means that you don't get enough energy anywhere to break the net. It just stretches as it decelerates the hail. You can see how this works by watching a soccer game and observing how the nets at the goals stop balls. Hailstones that weigh more than a soccer ball are extremely rare, as are hailstones that are falling faster than many strikes and penalty kicks in a professional soccer game, and you don't see many soccer balls breaking through the net. People have calculated how fast a soccer ball would have to go to get through the net and found that it is over 220 mph. The largest recorded hailstone is estimated to have had a terminal velocity of 168 mph, based on size, mass, and atmospheric conditions. That hailstone had about 16% more kinetic energy than a 220 mph soccer ball and so might have broke the net, but soccer nets are by no means the strongest nets that can made. And remember that was the largest hailstone ever recorded--we are talking a once in decades event.

Calwestjobs 1y ago

i used standard chain link fence on top of my small greenhouse and it works even with such big "holes", no mechanism, just manual labor but needs good forecast which can be issue.

nbadg 1y ago
Chain link fence is a really clever solution!
- Calwestjobs 1y ago
  well, makes goats not eat precious plants so if it can withstand them, then hail will be ez. XD (neighbors goats. i do not like animals, too much worry and work around diseases. there are tens, some years even hundred millions of chicken burned because of avian flu, pigs have african swine fewer,.... no thank you, corona cave bats of 2019, was enough for me...)
kyboren 1y ago
> it works even with such big "holes" I'm intrigued. Can you expand more on this? Why does it work? I guess because the hail is rarely coming straight-on at the holes and instead makes more glancing blows off the wire? In any case I think it's a perfect example of a good hack. Original out-of-the-box thinking producing an effective but unconventional solution. Chapeau bas!
- Calwestjobs 1y ago
  i saw chicken wire used for something unknown to me on top of old 1800s greenhouses in France. it was not used for hail because it was resting directly on top of glass. i do not know what it was there for. but hail and flying debris i got past few years is so big and so frequent that im not sure chicken wire will do the job. a lot of gardeners use shading cloth for lowering sun light intensity. but that is not always enough to withstand hail. A lot of times there is multiple pieces of hail stuck in one hole. some just bounce off. i have PV on roof and not had issue with that. but i had few broken glass pieces of greenhouse glass. even tho it is on same property. it is glass-glass bifacial, i did not do it for back light but for longevity, i saw too many plastic backing tear off, or delaminated or how to call it. so i did not want that. and im not sure if that is also factor in not having broken PV. doing it on huge PV areas should atleast be by my feeling too much of hassle, investment. But some people mentioned in other comments small arrays of, if i remember correctly, 5kWp. so that is roughly area im doing it on. so in those small arrays it can be done manually. https://99percentinvisible.org/article/fruit-walls-before-greenhouses-walled-gardens-created-urban-micro-climates/ https://99percentinvisible.org/article/fruit-walls-before-gr... https://www.atlasobscura.com/adventures/trips/peru-machu-picchu https://www.atlasobscura.com/adventures/trips/peru-machu-pic...